1. Field
This disclosure relates generally to audio amplifiers, and more specifically, to a method and apparatus for generating a discrete noise-shaped variable switching frequency signal.
2. Related Art
In class D audio amplifiers, digital pulse width modulation (PWM) can be used to convert a digital signal into a series of pulses based on a switching frequency. The switching frequency can be quantized by a high frequency system clock. The switching frequency is typically fixed, which produces discrete tones in the signal spectrum located at the switching frequency and its harmonics. These discrete tones are undesirable since they can produce electromagnetic interference (EMI). In communication applications, the discrete tones may exceed EMI regulations. Furthermore, in audio amplifier applications used in conjunction with a radio system (e.g., an AM or FM radio), the discrete tones can interfere with the receive channel distorting the information to be amplified, which is undesirable.
Various methods have been used to reduce EMI. For example, low pass filters have been used to reduce EMI. However using low pass filters in amplifiers can increase the size and cost of the amplifiers, which is undesirable. Another method to reduce EMI is frequency dithering. However, frequency dithering is more suitable for analog PWM since the switching frequency can be varied in a continuous fashion. In digital PWM, the switching frequency is based on counting the system clock and therefore the switching frequencies are discrete and tonal. Moreover, in class D audio amplifier applications, the high frequency system clock may be generated from a high accuracy low noise crystal reference, and performing frequency dithering on the reference clock may produce noise in the audio band.
Although dithering the switching frequency can effectively reduce overall EMI amplitudes, it broadens the frequency range of spectral content increasing the range of possible radio frequency interference, which is undesirable.